I make black mmf by dumping and entire jar of the wilton black gel in with the melted marshmallows as I add it them to the sugar. Let it sit over night and it will deepen. I use this mostly for accents. If I need to cover an entire cake black I usually cover it white and then airbrush so that its not tooth stainging (just a thin coating of food coloring instead of all through)HTH

If you add some cocoa powder in when you're adding in your powdered sugar it will give you a darker base to work with. But I always add in a whole lot of black and like momma said, letting it sit over night will make it darker.

Use a lot (1/2 to 3/4 cup) of cocoa powder in place of the powdered sugar, don't forget to put some vanilla flavoring to counter act the bitterness of the cocoa and you won't have to add much black color to the MMF to get the right color. Then let it sit for a couple of days to let the color ripen.